1. Field of the Invention
This invention relates to a rubber composition. More particularly, it relates to a rubber composition having an increased loss factor (tan .delta.) without impairing the mechanical strength.
2. Description of the Related Art
In recent years, the progress in high-performance automobiles, the extension of paved roads and the growth of superhighway networks have increased the demand to improve the gripping performance of tires, which performance has a close relation to acceleration capability and braking capability of automobiles. It is known that a high gripping performance can be achieved if the energy loss due to the friction between a tire tread and a road surface is increased. Because of this, there have been desired tread rubber materials which have an increased loss factor (tan .delta.) upon defomation.
It has also been desired to improve the damping capacity and the vibration-insulation property of damping materials, in particular, rubber vibration isolators for use in automobiles and home electrical appliances, so as to attain improved comfortableness and quietness. The properties required for rubber vibration isolators are:
(1) to have a hardness higher than a certain level because of necessity for carrying huge static loads; PA1 (2) to have a low vibration transmissibility, more specifically to have a high loss factor (tan .delta.) for vibration in a low frequency region, namely during idling and low speed driving, and at the same time to have a low dynamic-to-static modulus ratio (dynamic modulus of elasticity/static modulus of elasticity) for noise and vibration in a high frequency region, namely during high speed driving; and PA1 (3) to be excellent in resistance to flexural fatigue and durability against external force repeatedly applied for a prolonged period of time.
It is also important that the static fatigue resistance, such as compression set, is not inferior to that of ordinary rubbers.
Among hitherto known techniques for increasing the loss factor (tan .delta.) upon deformation are included the one in which a styrene-butadiene copolymer rubber having a high styrene content is used as a base, the one in which a large quantity of process oil is incorporated, and the one in which a highly reinforceable carbon black is added in a large quantity. In Japanese Patent Kokai (Laid-open) No. 70,539/89, it is disclosed to improve the loss factor of a rubber by blending the rubber with a (meth)acryloyl-containing p-phenylenediamine derivative, such as N-methacryloyl-N'-phenyl-p-phenylenediamine.
However, the use of a styrene-butadiene copolymer rubber having a high styrene content is still insufficient, though it shows some improvement in the loss factor. The addition of a large quantity of process oil or highly reinforceable carbon black can improve the loss factor, but it brings about an increase in heat build-up of the rubber to deteriorate the strength characteristics and abrasion resistance of the rubber, and also brings about significant increase in a dynamic-to-static modulus ratio. The use of the (meth)acryloyl-containing p-phenylenediamine derivative disclosed in Japanese Patent Kokai (Laid Open) No. 70,539/89 can increase the loss factor of the rubber, but it has become clear to have a drawback in that the strength characteristics of the resulting rubber compositions tend to be deteriorated.